From DE-PS 25 03 143 a high-speed forging press is known that is driven by continuously variable reversible-flow pumps having a controlled working stroke that is sinusoidal in the reversing phases (modified sine drive). This forging press has a retraction drive that is independent of the main drive and has a constant retraction force, a reversible decompression valve connected in parallel to the drive pumps and a control unit to open the decompression valve at the lower reversal point of the press stroke. The object of this is to obtain optimum freedom from surging for the hydraulic system of the oil-hydraulic drive despite the high operating speed. A contribution is made to this by special dimensioning of the opening area of the decompression valve, namely such that the pressure relief of the working cylinder of the press occurs partly through the decompression valve and partly through the drive pumps that are reversed to suck the pressure medium out of the working cylinder.
Meanwhile similar units have been built that also have retraction systems that make use of the reversed flow of one or more main pumps so as to avoid the need for control valves in the main circuits.
The above-mentioned press unit is a forging press with at least one integrated manipulator that runs on rails and in which the stroke movement of the press and the feed movement of the manipulator that guides the workpiece through the press are coordinated so that a workpiece with a predetermined form can be obtained.
In all forging presses of conventional construction the lower die of the press is fixed during the forging process, while the upper die carries out the working movement. As a result, however, the middle axis of the workpiece is unavoidably moved downwards in each working stroke during the shaping of the workpiece. The forging manipulator has to follow this displacement of the axis of the workpiece, which can be done for example through resilient mounting of the tong arm, as otherwise distortion of the workpiece cannot be avoided.
A wide variety of such resilient mountings, based on mechanical or hydraulic elements, are known, e.g. from DE-AS 22 55 009, DE-OS 15 27 354 or DE-GM 86 200 700.8. To reduce the bending forces acting on the workpiece as far as possible, the restoring forces of the springs are adapted to the load or load moment of the forging manipulator. This so-called passive springing cannot however provide completely trouble-free operation when forging rods, since on striking the workpiece the press accelerates the mass of the workpiece and the mass of the tong system downwards, with a lever action that varies with the distance of the manipulator from the press (i.e. as the length of the workpiece increases). Through the forces thus set up the workpiece is distorted and sometimes remains hanging on the edges of the die, which greatly interferes with the feeding of the workpiece.
Instead of using a resilient mounting it is also known to perform a controlled lowering or stroke movement of the tong arm (see DAS 1 296 117, DOS 1 527 261 and DAS 1 627 414). It is found, however, that satisfactory results cannot be obtained in this way, this being connected with the fact that in the case of forging presses with valve control the working speeds are normally very variable and the switching time needed for the tracking by the forging manipulator or its manipulator elements.
A hydraulic coupling between press and manipulator can--at any rate theoretically--be obtained with the forging apparatus according to DAS 1 627 414. This coupling, however, has disadvantages that arise from the great length of the connecting line needed between the auxiliary cylinder unit employed in the press and the lowering cylinder unit of the manipulator. Since the manipulators sometimes have to travel distances as long as 23 meters and more, a rapid working movement of the press gives rise to compression and decompression surges in the connecting line, which because of the travel of the manipulator has to be a hose, which bring the system out of its synchronous rhythm.